Introduce a base pointer field in FixedTypedArrayBase and teach GC about it

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 361b4d91d7ffbdbf128fb05f6dde00e27f84e0a7..ea6c06d68c64193b38b19bd4e46550a3b62915a2 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -2421,6 +2421,12 @@ class ScavengingVisitor : public StaticVisitorBase {
                                              HeapObject* object) {
     int object_size = reinterpret_cast<FixedTypedArrayBase*>(object)->size();
     EvacuateObject<DATA_OBJECT, kWordAligned>(map, slot, object, object_size);
+
+    MapWord map_word = object->map_word();
+    DCHECK(map_word.IsForwardingAddress());
+    FixedTypedArrayBase* target =
+        reinterpret_cast<FixedTypedArrayBase*>(map_word.ToForwardingAddress());
+    target->set_base_pointer(target, SKIP_WRITE_BARRIER);
   }
 
 
@@ -2428,6 +2434,12 @@ class ScavengingVisitor : public StaticVisitorBase {
                                                HeapObject* object) {
     int object_size = reinterpret_cast<FixedFloat64Array*>(object)->size();
     EvacuateObject<DATA_OBJECT, kDoubleAligned>(map, slot, object, object_size);
+
+    MapWord map_word = object->map_word();
+    DCHECK(map_word.IsForwardingAddress());
+    FixedTypedArrayBase* target =
+        reinterpret_cast<FixedTypedArrayBase*>(map_word.ToForwardingAddress());
+    target->set_base_pointer(target, SKIP_WRITE_BARRIER);
   }
 
 
@@ -3908,6 +3920,7 @@ AllocationResult Heap::AllocateFixedTypedArray(int length,
 
   object->set_map(MapForFixedTypedArray(array_type));
   FixedTypedArrayBase* elements = FixedTypedArrayBase::cast(object);
+  elements->set_base_pointer(elements, SKIP_WRITE_BARRIER);
   elements->set_length(length);
   if (initialize) memset(elements->DataPtr(), 0, elements->DataSize());
   return elements;